MOJAVE, Calif. — British billionaire Richard Branson showed off a key piece of his fledgling commercial space program Monday, unveiling a carrier aircraft designed to launch a passenger-carrying spaceship.

A crowd of engineers, dignitaries and space enthusiasts gathered inside a Mojave Desert hangar for the unveiling countdown. As the hangar door flew open, White Knight Two appeared outside under the sunny desert sky with Branson and American aerospace pioneer Burt Rutan waving from the cabin.

White Knight Two, billed as the world's largest all-carbon-composite aircraft, is "one of the most beautiful and extraordinary aviation vehicles ever developed," Branson said.

The public showing was the first concrete evidence of progress since the Rutan-designed SpaceShipOne became the first private manned rocket to reach space in 2004. After the groundbreaking flights, Rutan's Scaled Composites and Branson's Virgin Galactic partnered to commercialize on the success. White Knight Two and SpaceShipOne's successor, dubbed SpaceShipTwo, are being built by a joint venture called The Spaceship Company.

White Knight Two must undergo a rigorous flight testing program, beginning in the fall. Engineers still need to finish building SpaceShipTwo, which is now about 70 percent complete, according to Virgin Galactic.

Named after billionaire's mother
The mothership is a white, four-engine jet with room between its twin fuselages where SpaceShipTwo will be mounted for the flight to launch altitude. Virgin Galactic christened it Eve after Branson's mother, and the aircraft's side has a motif of a helmeted blond woman flying a Virgin flag.

White Knight Two has a 140-foot wingspan, about the same as a Boeing B-29 Superfortress, the World War II long-range heavy bomber.

White Knight Two is designed to cradle SpaceShipTwo under its wing and release it at an altitude of 50,000 feet. Once separated, SpaceShipTwo will fire its hybrid rocket and climb more than 62 miles (100 kilometers) above Earth.

From that height, which marks the internationally recognized boundary of outer space, up to six passengers will be able to float weightless for several minutes and see the curving Earth beneath a black sky.

When will space tourists fly?
The SpaceShipTwo effort is considered the most advanced and best-funded effort to send tourists on a suborbital ride into space. Other ventures working on suborbital spacecraft include Blue Origin (funded by Amazon.com billionaire Jeff Bezos), XCOR Aerospace (which is headquartered just down the street from Scaled Composites in Mojave) and Oklahoma-based Rocketplane Global.

Most of those ventures are targeting 2010 for the start of commercial service. However, the development timelines have slipped repeatedly and may well do so again. Branson has said as many as 100 test flights would be flown to make sure the SpaceShipTwo system is safe.

A year ago, Scaled Composites' development effort was dealt a blow when a nitrous oxide tank blew up during ground testing, killing three workers. State safety regulators hit the company with
a $28,870 fine
, which is being appealed.

Morgan said that Scaled has changed its procedures and that rocket engine development was back on track, but he declined to go into detail on Monday. Will Whitehorn, president of Virgin Galactic, said Scaled would post its findings online in a week or so.

Extra lift capacity
Whitehorn noted that White Knight Two was built to lift 30 percent more weight than a fully loaded SpaceShipTwo. It could even launch a vehicle capable of putting a single person into orbit, although that is "not a journey I think I'd probably take myself," he told msnbc.com.

That extra capacity means White Knight Two could fly from Mojave, or from Virgin Galactic's eventual base of operations in New Mexico, to other locations in the United States for SpaceShipTwo's launch. It could also make multiple hops and take on spaceflight passengers in other parts of the world. Talks are under way for space operations in Sweden, Scotland and Spain, Whitehorn said.

Eventually, White Knight Two could be used for much more than launching spaceships: Whitehorn suggested that the mothership could carry engines for delivery to aircraft manufacturers, or drop water for firefighting operations, or launch unmanned aerial vehicles.

"The applications of this vehicle ... are going to be many," Whitehorn said.

This report includes information from The Associated Press' Alicia Chang in Mojave and msnbc.com's Alan Boyle in Oshkosh, Wis.